Conventional space solar cells used in current space applications have an energy conversion efficiency of approximately 30%. Although, the efficiency of space solar cells is high as compared to terrestrial solar cells, which have an efficiency of approximately 15%, it is desired to increase the energy conversion efficiency of space solar cells to approximately 40 to 50%.
Conventional space solar cell technology achieves the 30% energy conversion efficiency by growing sequential layers of three different semiconductor materials, where each semiconductor material has a different bandgap. As a result, each different semiconductor material responds more efficiently to a different portion of the solar spectrum. Increasing the efficiency to the desired 40 to 50% requires increasing the number of layers in the solar cell. This, however, is a difficult task.
One conventional growth method is the lattice-matched growth method, where materials, typically III-V materials, are grown on top of one another to form a stack. In this method, however, the materials that can be utilized are limited because each layer in the stack must have the same crystal lattice, with the same atomic spacing. Thus, the efficiency of the solar cell is limited due the limited choice of materials.
Another conventional growth technique, referred to as inverted metamorphic (IMM) growth, allows the crystal lattice constraint to be somewhat relaxed. The downside, however, is that some defects are introduced into the last crystalline layer grown. In addition, once the cell is grown, the cell must be removed from the substrate on which it is grown. The reason for this is that the cell must be inverted to place the most-defective material (the last layer grown) on the bottom rather than on the light-facing side of the final cell. Other disadvantages with this technique are that that amount of lattice mismatch is limited and the mismatched layers must be kept thin. This results in an extremely thin and fragile cell. Consequently, the cell cannot be fully optimized for highest performance.
Still yet another conventional method to make solar cells with an increased efficiency would be to grow semiconductor layers on two different wafers and then bond the two wafers together to form a multi junction cell. This method permits the use of different semiconductor materials without a constraint on the lattice constant. For example, silicon, which is a low-cost material, can be used as a wafer for the bottom cell. Conventional bonding methods, however, to bond the wafers together to form the multi-junction cell employ a transparent polymer or glue to affix the wafers together. The transparent polymer or glue has a low refractive index in the range of approximately 1.3 to 1.5. As a result, this produces a large amount of reflection at each interface, because the reflection coefficient is proportional to the square difference of the refractive indices of the materials.